SyncValsverifier → artifact → classifier → verdict
SyncVals · Trajectory

byte-serial-round-scheduler

claude-code claude-opus-4-8 ✗ failed GOOD_FAILURE ↑ View task
Solved from the instruction alone, tests/ and solution/ were withheld from the agent's workspace and restored only for grading.
Reward = tests/test.sh exit code (0 → resolved); the classification below is post-hoc and cannot change it.
Classification , post-hoc; cannot change the reward
GOOD_FAILUREHonest miss, the agent ran correctly but couldn't solve it. Expected for a hard task; the task is sound.
SubtypeImplementation Bugs
EvidenceVerifier ran normally: dut_present/dut_forbidden/dut_elaborates/dut_compiles/simulation_completed all PASS, then reference_equivalence FAIL with "8892 mismatches in 16537 samples". The agent's DUT set next-round feedback with `start_fb <= reseed ? {4'b0, reseed_round} : {4'b0, round_index};`, while the reference solution uses `else fb <= {4'b0, rnd + 4'd1};`. The agent also did not clear `result_valid` every non-reset cycle, whereas the reference has `result_valid <= 1'b0;` before the FSM case.
Root causeThe task specification and tests are coherent behavioral checks for the described block engine. The agent made RTL interpretation/implementation errors around next-round feedback and output pulse behavior, so its compiled design mismatched the reference on many samples.
RecommendationN/A - task is fine
Trajectory
Tool-by-tool agent trajectory
4 tool calls · 2 tool types · 9 steps
A block engine transforms a 128-bit block under a start/ready protocol. Ports (1 bit unless width shown): clk rst_n start block_in 128 round_key0 128 num_rounds 4 reseed reseed_round 4 ready result_valid block_out 128 rst_n is active-low synchronous; while low at a rising edge outputs clear (ready high, result_valid low). start, while ready is high, loads block_in, round_key0 and num_rounds (1-15), else ignored; ready stays low until the finishing key-mix slot. Engine state is 128 bits, seeded by block_in. A round is sixteen byte slots then one key-mix slot. Byte slots visit columns 0 to 3; column c is state bits [32*c+31:32*c], its [31:24] byte first down to [7:0] last. A slot replaces only its byte with rol1(byte xor feedback) xor 8'h63; slot 0's feedback is the round's start feedback, else the previous slot's byte. round_index counts completed rounds. Round 0's start feedback is {4'b0, round_index}; a later round's is fixed at its preceding key-mix slot by reseed there (not reseed during the round): high gives {4'b0, reseed_round}, low {4'b0, round_index}. The key-mix slot xors state with round_key0. On the slot completing num_rounds, block_out becomes that post-xor state, result_valid and ready asserting that cycle; otherwise the next round starts next edge. block_out holds between operations, changing only on a finishing slot. All outputs are registered. Submit plain ASCII Verilog; $ system tasks and define/include directives are rejected. Write TopModule to /app/dut.sv.
I'll analyze the spec carefully and implement the block engine as a per-slot FSM (one slot per clock), then write it to `/app/dut.sv`. Key design decisions from the spec: - **State machine**: idle → 16 byte-slot cycles → 1 key-mix cycle, per round. - **Byte visitation order**: for slot `s`, byte index = `{slot[3:2], ~slot[1:0]}` (column `s/4`, high byte first) → order 3,2,1,0, 7,6,5,4, ... - **Feedback chain**: slot 0 uses the round's start feedback; each later slot uses the previous slot's produced byte (continuous chain across columns). - **Start feedback**: sampled from `round_index` (the completing round's index at the key-mix slot) or `reseed_round` when `reseed` is high there; round 0 = `{4'b0,0}`.
write · /app/dut.sv
module TopModule (
    input  wire         clk,
    input  wire         rst_n,
    input  wire         start,
    input  wire [127:0] block_in,
    input  wire [127:0] round_key0,
    input  wire [3:0]   num_rounds,
    input  wire         reseed,
    input  wire [3:0]   reseed_round,
    output reg          ready,
    output reg          result_valid,
    output reg  [127:0] block_out
);

    // Engine registers
    reg [127:0] state;
    reg [127:0] rkey;
    reg [3:0]   nrounds;
    reg [3:0]   round_index;   // number of completed rounds / current round index
    reg [4:0]   slot;          // 0..15 byte slots, 16 = key-mix slot
    reg [7:0]   start_fb;      // current round's start feedback
    reg [7:0]   prev_byte;     // previous slot's produced byte
    reg         busy;

    // Combinational datapath for the current byte slot
    wire [3:0]   bidx  = {slot[3:2], ~slot[1:0]};           // byte index within 128-bit word
    wire [6:0]   base  = {bidx, 3'b000};                    // bidx * 8
    wire [7:0]   cur   = state[base +: 8];                  // current byte at this slot
    wire [7:0]   fb    = (slot == 5'd0) ? start_fb : prev_byte;
    wire [7:0]   xr    = cur ^ fb;
    wire [7:0]   newb  = {xr[6:0], xr[7]} ^ 8'h63;          // rol1(byte^fb) ^ 0x63
    wire [127:0] mixstate = state ^ rkey;                   // key-mix result

    always @(posedge clk) begin
        if (!rst_n) begin
            ready        <= 1'b1;
            result_valid <= 1'b0;
            block_out    <= 128'b0;
            busy         <= 1'b0;
            slot         <= 5'b0;
            round_index  <= 4'b0;
        end else begin
            if (!busy) begin
                if (start) begin
                    // Load a new operation (ready is high here)
                    state        <= block_in;
                    rkey         <= round_key0;
                    nrounds      <= num_rounds;
                    round_index  <= 4'b0;
                    slot         <= 5'b0;
                    start_fb     <= 8'b0;          // {4'b0, round_index=0}
                    prev_byte    <= 8'b0;
                    busy         <= 1'b1;
                    ready        <= 1'b0;
                    result_valid <= 1'b0;
                end
                // else: stay idle; ready/result_valid/block_out hold
            end else begin
                if (slot < 5'd16) begin
                    // Byte slot: replace only its byte
                    state[base +: 8] <= newb;
                    prev_byte        <= newb;
                    slot             <= slot + 5'b1;
                end else begin
                    // Key-mix slot
                    if (round_index + 4'b1 == nrounds) begin
                        // Finishing slot of the operation
                        block_out    <= mixstate;
                        result_valid <= 1'b1;
                        ready        <= 1'b1;
                        busy         <= 1'b0;
                        slot         <= 5'b0;
                    end else begin
                        // Advance to next round; fix its start feedback here
                        state       <= mixstate;
                        round_index <= round_index + 4'b1;
                        start_fb    <= reseed ? {4'b0, reseed_round}
                                              : {4'b0, round_index};
                        slot        <= 5'b0;
                    end
                end
            end
        end
    end

endmodule
Let me verify it compiles with a quick syntax check if a Verilog simulator is available.

Check Verilog compiles

shell
(which iverilog && iverilog -g2012 -o /tmp/dut_test dut.sv -s TopModule 2>&1 && echo "COMPILE_OK") || echo "iverilog not available"
output
/usr/bin/iverilog
COMPILE_OK

[stdout]
/usr/bin/iverilog
COMPILE_OK
Compiles. Let me validate the timing/algorithm against an independent reference model.

Run simulation testbench

shell
cat > /tmp/tb.sv <<'EOF'
module tb;
  reg clk=0, rst_n=0, start=0, reseed=0;
  reg [127:0] block_in, round_key0;
  reg [3:0] num_rounds, reseed_round;
  wire ready, result_valid;
  wire [127:0] block_out;
  TopModule d(.clk(clk),.rst_n(rst_n),.start(start),.block_in(block_in),
    .round_key0(round_key0),.num_rounds(num_rounds),.reseed(reseed),
    .reseed_round(reseed_round),.ready(ready),.result_valid(result_valid),.block_out(block_out));
  always #5 clk=~clk;
  task run; begin
    @(posedge clk); #1; start=1; @(posedge clk); #1; start=0;
    wait(result_valid && ready);
    @(posedge clk); #1;
    $display("RESULT %032x", block_out);
  end endtask
  initial begin
    rst_n=0; @(posedge clk); #1; rst_n=1;
    block_in=128'h00112233445566778899aabbccddeeff;
    round_key0=128'h0f0e0d0c0b0a09080706050403020100;
    num_rounds=1; reseed=0; reseed_round=0; run;
    num_rounds=4; reseed=0; run;
    num_rounds=4; reseed=1; reseed_round=4'd9; run;
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim /app/dut.sv /tmp/tb.sv && vvp /tmp/sim
output
RESULT c6dc8f0c1feb655ddfe7f0fbf92fe555
RESULT 19c8299774cd36782015430aee1999a8
RESULT fe03a49c0a71eec8c7dece0190a54118
/tmp/tb.sv:24: $finish called at 1626 (1s)

[stdout]
RESULT c6dc8f0c1feb655ddfe7f0fbf92fe555
RESULT 19c8299774cd36782015430aee1999a8
RESULT fe03a49c0a71eec8c7dece0190a54118
/tmp/tb.sv:24: $finish called at 1626 (1s)

Python reference model

shell
python3 <<'EOF'
def rol1(b): return ((b<<1)|(b>>7))&0xff
def engine(block, key, nrounds, reseed=0, reseed_round=0):
    state=list(block.to_bytes(16,'big'))  # state[0]=MSByte = bits[127:120] = byte index15
    # our byte index k = bits[8k+7:8k]; big-endian list index i -> byte number 15-i
    # build access by byte-number
    def get(bn): return state[15-bn]
    def setb(bn,v): state[15-bn]=v
    round_index=0
    start_fb=0  # {0,round_index=0}
    for r in range(nrounds):
        prev=0
        for s in range(16):
            bidx=( (s>>2)<<2 ) | (3-(s&3))   # 4*(s/4)+(3-s%4)
            cur=get(bidx)
            fb=start_fb if s==0 else prev
            nb=rol1(cur^fb)^0x63
            setb(bidx,nb); prev=nb
        # key-mix
        v=int.from_bytes(bytes(state),'big')^key
        state=list(v.to_bytes(16,'big'))
        if round_index+1==nrounds:
            return v
        else:
            start_fb= reseed_round if reseed else round_index
            round_index+=1
    return int.from_bytes(bytes(state),'big')

bi=0x00112233445566778899aabbccddeeff
k =0x0f0e0d0c0b0a09080706050403020100
for nr,rs,rr in [(1,0,0),(4,0,0),(4,1,9)]:
    print("%032x"%engine(bi,k,nr,rs,rr))
EOF
output
c6dc8f0c1feb655ddfe7f0fbf92fe555
19c8299774cd36782015430aee1999a8
fe03a49c0a71eec8c7dece0190a54118

[stdout]
c6dc8f0c1feb655ddfe7f0fbf92fe555
19c8299774cd36782015430aee1999a8
fe03a49c0a71eec8c7dece0190a54118
The DUT matches an independent reference model on all three vectors , single round, multi-round, and the reseed path. The implementation is written to `/app/dut.sv`. Summary of the design: - **FSM**: one clock per slot , 16 byte-slot cycles then 1 key-mix cycle per round; a load cycle kicks off when `start & ready`. - **Byte order**: slot `s` targets byte index `{slot[3:2], ~slot[1:0]}`, i.e. column `s/4` high-byte-first → 3,2,1,0, 7,6,5,4, 11,10,9,8, 15,14,13,12, matching "column c = bits [32c+31:32c], [31:24] first". - **Slot transform**: `rol1(byte ^ feedback) ^ 8'h63`; feedback is `start_fb` at slot 0, otherwise the previous slot's produced byte (chained continuously across columns). - **Start feedback**: round 0 = `{4'b0, 0}`; each later round's is fixed at the preceding key-mix slot from `reseed_round` (if `reseed` there) or the current `round_index`. - **Key-mix**: `state ^ round_key0`; on the slot completing `num_rounds`, `block_out`, `result_valid`, and `ready` all assert that cycle. `block_out` holds otherwise. - **Reset**: synchronous, active-low → `ready` high, `result_valid` low, outputs cleared. - All outputs registered; no `$` tasks or `` `define``/`` `include`` directives. One spec ambiguity I resolved deliberately: the reseed-low start feedback uses the `round_index` value present *at* the preceding key-mix slot (the completing round's index), reading the spec's `{4'b0, round_index}` literally. If the intended reference instead meant the upcoming round's index (`round_index + 1`), that single line is the one to flip.
Final diff , what the agent changed

No file changes, the agent left the workspace untouched.

Verifier output , the proof behind the reward
tests/test.sh
TEST dut_present: PASS (/app/dut.sv exists)
TEST dut_forbidden: PASS (no forbidden constructs)
TEST dut_elaborates: PASS (DUT elaborates standalone)
TEST dut_compiles: PASS (compiled with held-back reference/testbench)
TEST simulation_completed: PASS (Mismatches: 8892 in 16537 samples)
TEST reference_equivalence: FAIL (8892 mismatches in 16537 samples)
{"score": 0.000000, "results_file": "/tmp/h2t-grader-home.OjOJMs/runtime/cb_verifier_317-19801-10573-32634_317/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_42b0ab7abb25446a. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_42b0ab7abb25446a · verifier authoritative; classifier explanatory.